What? .. and no psu? no hdd/ssd?
Anyway - I'd upgrade GPU first and see, how that goes. If graphics quality settings you're after, then gpu upgrade will provide that.
CPU will hold down your system - that's for sure. But - to what extent?
You'll have to check and see, how that upgraded GPU performs in your system.
If performance still is not satisfactory, then start saving for a new cpu/motherboard/ram.
BTW - get SSD for your system, if you don't have one already.